rusty transformers a good thing

[gui_tarzan]

What a ridiculous article. It reads like it was written by kids in junior high school.

If a transformer has that kind of rust on it it's a good bet that the rest of the amp and parts have rust and/or corrosion in places it shouldn't. A Sunn head I picked up last winter was like that. I stripped the chassis and lightly sanded it where it was rusty, then cleaned it with DeOxIt. The jacks were corroded but fortunately the pots cleaned up pretty well. I replaced the broken bells on the transformers and spritzed some high-heat black on the laminates.

Coating these with layers of paint will only lead to heat issues as the paint acts like an insulator unless it's the aforementioned type. As far as rust getting into the laminates, they should be coated with varnish and shouldn't rust on the inside anyway.

If it's nasty rusty with flakes and chunks coming off and the screws disintegrating, that's a different story. Otherwise check the leads and use it.

[Leo_Gnardo]

Phil_S and others who wish to wire-brush or similar: mostly you want to avoid bridging laminations with something conductive, tiny shards of steel, bits of steel wool, accumulations of iron powder, like that. Rub & scrub at your own risk.

[teemuk]

Look how ridiculous benefits are gained in efficiency; With a rusted transformer you gain an astouding improvement of

...wait for it...

0.24 watts!

I'm stunned. Not even quarter of a watt.

In fact, a sceptic in me might categorize such figures to ordinary component tolerances. Since this is definitely by no means new information (e.g. utilising an oxide film to insulate laminations is pretty standard manufacturing practice) I wonder why we don't see any of those -really- rusted transformers in any real world applications. I would imagine power companies would be interested in saving a buck or two, instead they immerse transformers in oil and seal them carefully to do the exact opposite: prevent corrosion.

I rememeber when this topic came out in 2009. I researched it a bit then and basically everyone can draw their conclusions from the tidbits I've learned:
- Rusted ferrite is way inferior in passing magnetic flux than non-corroded ferrite and the result of this is actually the exact opposite of MM's "discovery": Higher core losses and inefficiency
- Earliest patents concerning protecting transformers from rust and corrosion dated way back to 20th century. No, designers didn't push forward in deliberately rusting transformers, exact opposite happened. And no, in my book an oxide film coating is not the same thing as "traditional" rust.
- You find several patents and documents discussing how to manufacture the ferrite laminates (which are very prone to rusting) in methods that try to prevent rusting from taking place
- There was a lot of information (patents and publications) about corrosive properties of transformer oils. None of these sources referred to corrosion as "beneficial". In fact, all of these sources spoke against letting corrosion to happen
- You never see rusted transformers fitted to real world applications. Yes, they might rust in use and little bit of exterior rust won't instantly turn a transformer faulty, but generally when transformers are fitted in NEW they are never rusted or corroded in any way. In fact, not even Mercury Magnetics sells us rusted transformers or amps/amp kits with rusted transformers. Obviously their money isn't where their mouth is on this topic.

...Well, you only need to look at the source to know to which category to put these kinds of "adverticles". Secondly, "Vintage Guitar Magazine" is never my first source for factual information. I'm pretty sure there are journals that would actually discuss this topic with a bit more scientific approach. Peer reviewing might not hurt either.

[teemuk]

I really like their bit about that ugly ass power cable they sell because it has better tone.

I remember those.

One particular thing about that cable I found hillarious was that while they used thicker gauge wire the actual connectors in it still had inferior voltage and current ratings in comparison to few brand X donor IEC cables I picked randomly from a drawer where I have about 5 - 10 IEC cables "cannibalized" from all sorts of places.

I haven't had a reason to buy these things for years, you practically can get them free from just about everywhere. So I can sort of see why MM needs to advertise such product seriously but I still found it rather.... hmm... cheap. I generally don't like advertisements that start from assumption that the target consumer is a fool and gets treated as such.

What can you say... Mercury Magnetics in their usual biz again.

[Phil_S]

Phil_S and others who wish to wire-brush or similar: mostly you want to avoid bridging laminations with something conductive, tiny shards of steel, bits of steel wool, accumulations of iron powder, like that. Rub & scrub at your own risk.

Well...this is a worthy point, that shorting the lams is a problem and I'm not meaning to be argumentative. I just don't see it as anything but a remote possibility. I hope folks won't mind a bit of a detour on transformer construction as it might illuminate the discussion.

I've actually disassembled a couple of transformers. One was successfully repaired, the other not repairable. I'll observe the most difficult thing about disassembling a transformer is separating the lams. As I understand it, a properly constructed transformer is dipped in varnish and then baked. That varnish hardens. Separating the lams requires a razor blade and some careful prying (and not with a screwdriver.) Reassembly was done with the Sprayon product I mentioned earlier. I won't suggest it's as good as a proper dipping and baking, but it is what I can do with what I've got. (You don't want to deal with the aftermath of baking a varnish-dipped transformer in the kitchen oven...it's a $1200+ bake job because she needs a new oven when you're done.)

OK, I know that's a little long winded.

Here's my point. If the rust has already eaten into the varnish so far as to allow little specs of brushed rust in between the lams, that is quite a lot of damage. I want to suggest a transformer in that shape shouldn't be used.

[vibratoking]

I am benefitting from the discussion, so thank you for the facts.

The heart of the issue is that MM is not dealing in facts. You can't make a valid comparison of anything with a sample size of two. The comparison could be invalid for so many reasons, but you can't determine the reality because no tangible information is provided. My guess is that you could switch those power meters and the result would be different. Same goes for the temp measurement.

[Cliff Schecht]

Eh, I like 'em rusty just fine. Some transformers use varnish to insulate, others (typically older) use an oxide layer (but not necessarily iron oxide as this tends to "run"). As long as the laminates aren't shorting, I will knock off the loose rust and use it as is.

[teemuk]

We also need to bear in mind that rust will absorb moisture from atmosphere and gradually just keeps on spreading and spreading.

Little surface rust does no harm, but it is sure a sign that some day we can also expect to encounter more than "little surface rust".

[Phil_S]

I'm enjoying this. Let's look at a few pictures. This is the lam set from the tx that I couldn't repair. It was a PT with a shorted winding. Remember, lams are E's and I's.

This one has virtually no exterior rust. Look at the inside:

#2600 shows the 2nd outermost lam. You can see both rust and varnish.

#2602, shows a random inner E and I lam that's got just a little surface rust. There isn't much varnish if any on the lam beyond the edge. Honestly, I didn't realize these things aren't supposed to "connect" with the one next to is. I'm not sure I understand this. In any case, properly bolted together and varnishished, I remain unconvinced (re: Leo) that anything would get in there from a light brushing. I'm not talking about a hard steel wire brush with lots of elbow grease applied.

#2602, you can see some of the edge and it isn't rusty. This is representative of all 4 external faces.

#2604 shows rust on the inside window. How'd that get there? This has been in a dry place since I took it apart. I don't remember if rust was present when I took it apart.

[Cliff Schecht]

Interesting. I am curious myself if rust can get so bad as to start cracking the varnish and causing small shorts in the laminations.

Also this is why it's a good idea to cook your unknown vintage transformers before you drop them in if there are any doubts. The transformer shown above was probably usable if the winding wasn't shorted and furthermore I doubt any issues with the laminations caused it to fail (not sure if you mentioned the failure mechanism). I've got my fair share of rusty transformers in builds and am yet to have one of them fail. A visual inspection is usually enough to make me happy and it's easy to pull the shells off and look inside. If the rust has really penetrated then IME the wires can also be corroded and the transformer will either need some TLC or just won't get used in a build.

[Phil_S]

I can tell you, it was hard work to get the lams apart on that one. It was good n tight. I agree it would have been fine had it not had the short. It was acquired on a used chassis very cheap. I didn't feel bad, except my bargain wasn't. The short was unfortunately in the B+ winding which was very thin wire -- IIRC 39AWG -- and impossible to unwind without proper equipment. OTOH, filament windings were fat -- I was surprised at how fat, 16AWG and 18AWG and the number of turns was quite small as you might guess, about 20 turns for one and about 25 turns for the other -- basically one layer and they were on the outside. It was a simple 3 layer wind.